On Mon, 10 Aug 2009 11:44:52 -0700, Roy Lewallen
wrote:

I also did some experiments in the early '70s to see if CP would reduce
fading. I built a couple of types of omnidirectional CP antennas -- a
"skew planar", and a copy of a commercial FM BC antenna, for mobile use
with the local 450 MHz repeater.

A "halo" type of antenna? Some of the commercial broadcast FM
antennas are eliptical polarized. Most of the signal is horizontally
polarized, but there is a small vertical component in order to improve
performance in vehicles.

We tried several antennas at the repeater end. Something like this
one seemed to work best:
http://iris.nyit.edu/~sblank/VPFMfig5.gif
We had 4 elements with a coax cable phasing mess.

I soon discovered that as soon as I
placed the antenna over the top of the car, the polarization became
nearly linear. I've since learned that it's because of the nature of the
reflections from the ground plane, and it's easily seen with EZNEC+.
When I put the antenna far enough away from the car to minimize
reflections, the lowered gain offset any possible advantage. Overall,
they worked out worse than a conventional vertically polarized antenna.

I tried to use CP on both ends and eventually gave up. Thanks for the
explanation, but I have a different theory. The polarization changes
sense (direction) every times it's reflected. We standardized on RH
CP. When the RH CP signal hits the car, it is reflected as LH CP. If
the LH CP signal arrives at the repeater antenna, which is RH
polarized, they cancel. If it became linear, it would theoretically
only present a -3dB polarization loss, which is not huge.

It might have been interesting to try CP at the repeater, but that was
never done.

I can testify that it worked quite well for solving the specific
problem. We were trying to eliminate picket fencing (frequency
selective fading or Rayleigh fading). While there were some half
hearted experiments with various CP mobile antennas, the major effort
was at the repeater end. This was about 1971 so the technology used
was rather crude. One student was doing his senior project (reqd for
graduation) around this test. Several of us were enlisted to help.
When was in the land mobile radio biz many years later, I repeated the
tests with similar results.

We hung a thermal chart recorder onto the first limiter testpoint (on
a Motorola Sensicon T43 receiver) and plotted signal strength versus
time as a mobile drove through the problem area with the xmitter keyed
continuously. The test was repeated with various tower mounted
antennas. The linear antenna had more signal (gain) than CP, but also
had many more fades, what were far more pronounced. In all, I would
call it an improvement in quality, but not in range.

The problem with ground reflection ruining the circularity makes it very
difficult to achieve circular polarization for HF skip communication.

Well, I supplied several examples of commercial HF antennas that are
circularly polarized. I'm tempted to try building one, just to see
what works or breaks.

A second problem is that the majority of CP antennas, such as the
quadrature fed crossed dipole "turnstile", are circular only directly
broadside, and increasingly elliptical as you move away from that direction.

That's why high accuracy GPS antennas use choke rings at the antenna.
It widens the pattern so that it picks up more of the sky, but also
maintains some semblence of CP at the horizon.

Any interest in me scanning and posting the chapter on circular
polarization repeater antennas from the TAB book? 13 pages with some
low quality pictures.

Roy Lewallen, W7EL

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann wrote:
On Mon, 10 Aug 2009 11:44:52 -0700, Roy Lewallen
wrote:

I also did some experiments in the early '70s to see if CP would reduce
fading. I built a couple of types of omnidirectional CP antennas -- a
"skew planar", and a copy of a commercial FM BC antenna, for mobile use
with the local 450 MHz repeater.

A "halo" type of antenna? Some of the commercial broadcast FM
antennas are eliptical polarized. Most of the signal is horizontally
polarized, but there is a small vertical component in order to improve
performance in vehicles.

The "skew planar" looked like a cloverleaf antenna with each "petal"
rotated 45 degrees. The other was a copy of a broadcast antenna
advertised to be circularly polarized. I used a simple hand held dipole
and field strength meter to judge polarization. I know now it was
subject to a number of shortcomings, but I feel it did a reasonable job
of indicating circularity. Both antennas were reasonably circular.

. . .

I tried to use CP on both ends and eventually gave up. Thanks for the
explanation, but I have a different theory. The polarization changes
sense (direction) every times it's reflected. We standardized on RH
CP. When the RH CP signal hits the car, it is reflected as LH CP. If
the LH CP signal arrives at the repeater antenna, which is RH
polarized, they cancel. If it became linear, it would theoretically
only present a -3dB polarization loss, which is not huge.

No, that's a common misconception. A circularly polarized wave produces
a circularly polarized wave of the opposite handedness only when
reflected from a plane normal to its direction of propagation. That's
seldom the case in a communication environment. When reflected from
surfaces at other angles, the result is a change in circularity, from
elliptical to nearly linear depending on the angle of reflection and the
reflection coefficients of the surface. A short while with the modeling
program of your choice will confirm this.

. . .

A second problem is that the majority of CP antennas, such as the
quadrature fed crossed dipole "turnstile", are circular only directly
broadside, and increasingly elliptical as you move away from that direction.

That's why high accuracy GPS antennas use choke rings at the antenna.
It widens the pattern so that it picks up more of the sky, but also
maintains some semblence of CP at the horizon.

I wasn't aware of any GPS receivers using crossed dipole "turnstile"
type antennas. All the ones I've seen use either quadrifilar helix or
patch antennas. Can you point to a reference or two regarding the choke
rings -- I don't know what these are or what they do, and would like to
learn.

. . .

Roy Lewallen, W7EL

Roy/Jeff

This is all interesting stuff that I have always wanted to experiment on
some more. I note the comment (was it Roy?) about the positioning of a
CP antenna over a vehicle roof and the effect that had on the total
signal polarization. I slapped my forehead on that one! (ie I should
have known and tested for it)

I also used a skew planar loop. It was I guess maybe 400mm above the
vehicle roof. It had 4 loops in phase. The halo/3 leaf HP (clover) was
the same basic construction. At the time the results of H vs V really
astounded me. In my case OTH UHF performance was maybe 12db "better"
mainly from the reduction in flutter.

No, that's a common misconception. A circularly polarized wave produces
a circularly polarized wave of the opposite handedness only when
reflected from a plane normal to its direction of propagation. That's
seldom the case in a communication environment. When reflected from
surfaces at other angles, the result is a change in circularity, from
elliptical to nearly linear depending on the angle of reflection and the
reflection coefficients of the surface. A short while with the modeling
program of your choice will confirm this.

Yes of course! (slaps forehead again!)

This is something that has come out recently in some experiments I have
been doing with mobile data comms on VHF SSB. I have a badly written and
incomplete page;

http://pages.suddenlink.net/vk2yqa

Before getting into the data side of it I had noted that a lot of
intelligence could be gleaned by looking at the Doppler effect from the
moving vehicle. Only a tiny 30Hz or so was noted on 144MHz but it showed
to my mind that a lot of signal comes from multiplathing and even some
comes from double reflections. The level of shift over the Doppler
"bandwidth" also showed that it was fairly evenly distributed. ie
reflections off plane objects like oncoming vehicles were not really any
stronger than from other directions. Such things as large oil tanks
though are quite obvious when I correlate the trip timing with nearby
objects. This also bought sense to the experiments I did back in the
80's where I tried a 3 el quad on the vehicle with dissappointing results.

So where is this going? I wonder what the result would be Doppler
"bandpass" wise if I used a CP antenna on the vehicle and base now? (The
above mentioned results are all VP) Given the sense change reflections I
wonder if I would get a "null" in the received bandpass due to movement.
I am trying to visualize the result of this;

http://pages.suddenlink.net/vk2yqa/img1.png

during that test. Would I get a series of bands parallel to the
envelope edges that would further indicate direction of travel relative
to the base. I realize that the direct path already gives that info,
just trying to get the likely scenbario in my head.

Apologies to Art for taking his post so far OT.

Cheers Bob VK2YQA

On Aug 11, 4:47*pm, Bob Bob wrote:
Roy/Jeff

This is all interesting stuff that I have always wanted to experiment on
some more. I note the comment (was it Roy?) about the positioning of a
CP antenna over a vehicle roof and the effect that had on the total
signal polarization. I slapped my forehead on that one! (ie I should
have known and tested for it)

I also used a skew planar loop. It was I guess maybe 400mm above the
vehicle roof. It had 4 loops in phase. The halo/3 leaf HP (clover) was
the same basic construction. At the time the results of H vs V really
astounded me. In my case OTH UHF performance was maybe 12db "better"
mainly from the reduction in flutter.

No, that's a common misconception. A circularly polarized wave produces
a circularly polarized wave of the opposite handedness only when
reflected from a plane normal to its direction of propagation. That's
seldom the case in a communication environment. When reflected from
surfaces at other angles, the result is a change in circularity, from
elliptical to nearly linear depending on the angle of reflection and the
reflection coefficients of the surface. A short while with the modeling
program of your choice will confirm this.

Yes of course! (slaps forehead again!)

This is something that has come out recently in some experiments I have
been doing with mobile data comms on VHF SSB. I have a badly written and
incomplete page;

http://pages.suddenlink.net/vk2yqa

Before getting into the data side of it I had noted that a lot of
intelligence could be gleaned by looking at the Doppler effect from the
moving vehicle. Only a tiny 30Hz or so was noted on 144MHz but it showed
to my mind that a lot of signal comes from multiplathing and even some
comes from double reflections. The level of shift over the Doppler
"bandwidth" also showed that it was fairly evenly distributed. ie
reflections off plane objects like oncoming vehicles were not really any
stronger than from other directions. Such things as large oil tanks
though are quite obvious when I correlate the trip timing with nearby
objects. This also bought sense to the experiments I did back in the
80's where I tried a 3 el quad on the vehicle with dissappointing results..

So where is this going? I wonder what the result would be Doppler
"bandpass" wise if I used a CP antenna on the vehicle and base now? (The
above mentioned results are all VP) Given the sense change reflections I
wonder if I would get a "null" in the received bandpass due to movement.
*I am trying to visualize the result of this;

http://pages.suddenlink.net/vk2yqa/img1.png

during that test. Would I get a *series of bands parallel to the
envelope edges that would further indicate direction of travel relative
to the base. I realize that the direct path already gives that info,
just trying to get the likely scenbario in my head.

Apologies to Art for taking his post so far OT.

Cheers Bob VK2YQA

I don't see it as OT, if you get a response run with it
cheers

On Tue, 11 Aug 2009 13:13:38 -0700, Roy Lewallen
wrote:

A second problem is that the majority of CP antennas, such as the
quadrature fed crossed dipole "turnstile", are circular only directly
broadside, and increasingly elliptical as you move away from that direction.

Sorry, my original reply to this comment was screwed up thanks to me
talking on the phone while writing.

Yeah, the problem with turnstiles CP is at the horizon. At the
horizon, one element of the turnstile would be roughly perpendicular
to me, thus acting as a simple linear dipole. The other crossed
element would be seen from the end, resulting in no radiation in my
direction. So, at the horizon, a turnstile is mostly linear
polarization.

That's why high accuracy GPS antennas use choke rings at the antenna.
It widens the pattern so that it picks up more of the sky, but also
maintains some semblence of CP at the horizon.

I wasn't aware of any GPS receivers using crossed dipole "turnstile"
type antennas. All the ones I've seen use either quadrifilar helix or
patch antennas. Can you point to a reference or two regarding the choke
rings -- I don't know what these are or what they do, and would like to
learn.

If you really want a turnstile GPS antenna:
www.arrl.org/tis/info/pdf/0210036.pdf

As for the choke ring, you've probably seen them in the center of
C-band DBS/TVRO dish antennas.

Links:
http://www.javad.com/jns/index.html?/jns/technology/Choke%20Ring%20Theory.html
http://www.trimble.com/infrastructure/gnss-choke-ring-antenna.aspx?dtID=overview
http://www.gpsworld.com/survey/news/trimble-choke-ring-antenna-uses-dorne-and-margolin-dipole-3620

More detail:
www.novatel.com/Documents/Papers/3D_choke_ring.pdf
If you remove the radome in the center, it's a "pinwheel" antenna,
with which I'm totally unfamiliar. Note the comments on "low
elevation tracking", which is what I was mumbling about for improving
the performance at the horizon. The above article don't show it, but
the choke ring does maintain some semblance of CP near the horizon.

Patent on the dual frequency (L1 and L2 for GPS) choke ring:
http://www.google.com/patents/about?id=EiwIAAAAEBAJ&dq=6278407

On Tue, 11 Aug 2009 13:13:38 -0700, Roy Lewallen
wrote:

Jeff Liebermann wrote:
On Mon, 10 Aug 2009 11:44:52 -0700, Roy Lewallen
wrote:

I also did some experiments in the early '70s to see if CP would reduce
fading. I built a couple of types of omnidirectional CP antennas -- a
"skew planar", and a copy of a commercial FM BC antenna, for mobile use
with the local 450 MHz repeater.

A "halo" type of antenna? Some of the commercial broadcast FM
antennas are eliptical polarized. Most of the signal is horizontally
polarized, but there is a small vertical component in order to improve
performance in vehicles.

The "skew planar" looked like a cloverleaf antenna with each "petal"
rotated 45 degrees. The other was a copy of a broadcast antenna
advertised to be circularly polarized. I used a simple hand held dipole
and field strength meter to judge polarization. I know now it was
subject to a number of shortcomings, but I feel it did a reasonable job
of indicating circularity. Both antennas were reasonably circular.

Sounds reasonable. Incidentally, the FM broadcast "cloverleaf"
antenna was invented by Philip Smith, the inventor of the Smith Chart:
http://www.ieeeghn.org/wiki/index.php/Philip_H._Smith_Oral_History#FM_Broadcasting_and_t he_Cloverleaf_Antenna

I blundered cross this page on a 6/2 meter CP antenna design. Looks
workable but very narrow band:
http://www.wa7x.com/cycloid_info.html

I tried to use CP on both ends and eventually gave up. Thanks for the
explanation, but I have a different theory. The polarization changes
sense (direction) every times it's reflected. We standardized on RH
CP. When the RH CP signal hits the car, it is reflected as LH CP. If
the LH CP signal arrives at the repeater antenna, which is RH
polarized, they cancel. If it became linear, it would theoretically
only present a -3dB polarization loss, which is not huge.

No, that's a common misconception. A circularly polarized wave produces
a circularly polarized wave of the opposite handedness only when
reflected from a plane normal to its direction of propagation. That's
seldom the case in a communication environment. When reflected from
surfaces at other angles, the result is a change in circularity, from
elliptical to nearly linear depending on the angle of reflection and the
reflection coefficients of the surface. A short while with the modeling
program of your choice will confirm this.

With my limited abilities, it will probably take more than a "short
while" with an antenna modeling program. I've never done any
polarization studies. I'll take your word for this, but it would seem
that there would be a gradual transition from total reversal in sense
from a perpendicular reflector, to conversion to linear polarization
with shallow reflection angles. I'll see if I can find some research
on this. (I'm curious).

I once did some crude experimentation on the degree of isolation
provided by a reflective sense reversal. I just happen to have two
big 2.4Ghz helical antennas, both RH CP. I separated them by about 15
meters and measured the received signal level. I then placed an
obstruction (corner of building) along the line of sight, and supplied
a flat plate reflector. I didn't think to try varying angles of
incidence and reflection and just ran it at 45 degrees from the plane
of the flat plate reflector. The signal dropped about -15dB which I
guess is about all I could expect in an uncontrolled environment. The
loss would indicate that the signal was still substantially circularly
polarized at 45 degrees. I still have the helix antennas and can
repeat the test if necessary (and if I can find the time).

Thanks for the clarification.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Roy Lewallen wrote:

I wasn't aware of any GPS receivers using crossed dipole "turnstile"
type antennas. All the ones I've seen use either quadrifilar helix or
patch antennas. Can you point to a reference or two regarding the choke
rings -- I don't know what these are or what they do, and would like to
learn.

. . .

Roy Lewallen, W7EL


For high performance GPS receivers and measurements, it's important that
the apparent position of the antenna be be independent of the look angle
to the various satellites. Partly this is by making antennas with a
phase center that is look angle independent, partly this is by making
sure you're not receiving a combination of direct and reflected waves.

Remember that for precision GPS, what you're looking at is essentially
the carrier phase within a single chip time (about 100ns). The carrier
phase outside the correlator's time window doesn't contribute to the
observable measurement (because it's got a random 180 degree phase shift
superimposed on it).

So what you're really worried about is interference that causes an
apparent change in phase of the carrier (at 1.5GHz.. call it 20cm
wavelength). In precision GPS, you're talking millimeter scale
measurements, or, say, better than 1 degree of apparent phase shift. A
reflected signal that is 35 dB down is enough to get this sort of error.


The multipath from "distant" reflectors is fairly easily dealt with by
putting the antenna on a pole. Distant, here, means a few meters away..
farther than that, and the multipath signal's code phase is far enough
away that it doesn't contribute to the measurement. The chip length is
about 100 ns, or 30 meters. So, putting the antenna on a pole a few
meters up, with a plate underneath it that cuts off the "view" of the
ground closer than a few meters away guarantees that you won't see any
reflections from something closer than 20 meters path length different.

You also choose an antenna that has very little gain below several
degrees above the horizon.

But, just any old flat plate won't work, because you can have a creeping
wave propagate across the surface AND you don't want the plate itself to
reflect a signal. Solution: make a plate that is a RF "black hole" at
the frequency of interest.. it's a series of grooves that are carefully
designed to attenuate the reflected and evanescent wave propagating
across the surface (just like in a corrugated horn).

The most common design is by Dorne and Margolin, and I guarantee you've
seen these if you've seen surveyors doing GPS measurements. They're
about 2' in diameter with several wide grooves around a small conical or
hemispherical radome covering the actual antenna. Sometimes, the whole
thing is covered by a hemispherical radome.

http://facility.unavco.org/project_s.../antennas.html

Is a photo of a typical geodetic installation (sub-mm accuracies)

http://www.trimble.com/infrastructur...?dtID=overview

is the actual antenna.